Interpatient variation in expression of P-glycoprotein in the intestine has been shown to substantially contribute to the variability in the oral availability of cyclosporin A (1). A probe based test capable of quantitating P-glycoprotein would be very valuable in characterizing the genetic and non genetic factors that underlie interindividual variation in intestinal expression of P-glycoprotein and to further extablish the role of P-glycoprotein in the oral kinetics of other drugs. This study will directly test the hypothesis that orally administered fexofenadine (Allgra) can be used as an assay for P-glycoprotein function.